In the prior art special devices are known of suitable for recovering mechanical kinetic energy, or energy harvesting, using vibrating systems operating on fields of varying frequencies typically from about 1 hertz [Hz] to about a few kilohertz [kHz].
These known devices are technical applications able to exploit known physical or chemical properties, such as piezoelectric, electrostatic, electrostrictive or electrochemical properties of some particular materials or composites, with the aim of producing energy transducers able to convert kinetic-vibrational mechanical energy into electricity. The electricity thus generated can be used directly or stored in an electrochemical accumulator or battery.
Said devices can be advantageously used for energy recovery in various fields of application and from any mechanical system that emits or dissipates mechanical energy in the form of vibrational oscillations, such as: the floors of buildings, flyovers, cars, machinery (e.g. jack hammer), etc.; by placing said device in the point where it has the greatest breadth of vibrational oscillation in order to maximize the energy recoverable by said device and convertible into electricity.
A further sought after and advantageous application of these known technologies consists of constructing portable devices suitable for energy recovery able to operate autonomously, adhering firmly to various types of vibrating surfaces, in such a way as to recover the energy, transforming it into electricity to be stored subsequently for later use.
Said portable devices can be incorporated, for example, into traditional soles and insoles for footwear, the latter devices being capable of recovering the kinetic energy generated while walking, energy which is partly dissipated naturally in contact with the ground or other elements. Walking typically produces a mechanical vibration characterized by low oscillation frequencies, generally between about 0.5-10 Hz, and by high amplitude.
The process of producing electricity which the device of the present invention is based on consists of the recovery of mechanical energy or energy harvesting by means of the physical phenomenon of electrostriction of a flexible and electrically insulating material.
Constructive technical solutions of energy recovery devices using the principle of electrostriction provide for the production of a film of material preferably plastic polymer having a thickness traditionally between a few dozen and a few hundred micrometers [μm], said film having the function of a dielectric of a capacitor, generally being coated on both sides by two metal electrodes for example in gold (Au), on the surface of which the necessary electrical contacts can be made in order to configure the armatures of said electric capacitor.
According to a typical embodiment, taken from the technical solution of the composite material with electrostrictive properties for a mechanical energy recovery device of the present invention, on said film, generally rectangular in shape, said gold electrodes are deposited, one on each face, said electrodes being generally the same shape but a slightly different size to each other, so as to leave a thin, non-coated edge such as to prevent the flow of current between the two opposite faces.
Said film is then electrically charged via a power supply unit inside the sole of a shoe, and constrained at the two ends at the metatarsal portion of the foot, where, while walking, a mechanical tension is generated as a result of bending of the foot.
However, said known technical applications based on the electrostriction principle of materials have limitations and operating faults.
Theoretically, to make a film in electrostrictive material any electrically insulating materials, with elastic properties and able to assume a suitable shape, can be used as energy transducers. However, such materials must necessarily have characteristics such that, in practice, the type of material actually utilisable is limited. In particular, good mechanical tensile strength, compressive and bending properties as well as good elasticity are required. The materials possessing these characteristics are elastomeric polymers, which are therefore among the preferred materials for the production of energy recovery devices based on the electrostriction process.
Currently, a number of homo-polymers and copolymers are known of which generate energy through the same physical process as electrostriction, e. g. nylon and ter-polymers type P(VDF-TrFE-CTFE) poly-(vinylidenefluoride-trifluoroethylene-chlorotrifluoroethylene) or P(VDF-TrFE-CFE) poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene).
The main drawback of these polymeric materials is that they have a greater stiffness and therefore the cyclical deformation of mechanical stretching and relaxation, a mechanical process fundamental to the production of energy, is more difficult. This means that the effective application of these polymeric materials in an energy recovery device is less advantageous, since an overly rigid material can affect and inhibit a natural walk, turning the device from “passive” (an object that the user uses in normal physical activity without influencing it) to “active” (an object that the user perceives the presence of during exercise being forced to make an extra effort because of the same).
A further operating limit of these known elastomeric polymers is the fact that they generally have low values of the relative dielectric constant c r, reducing the amount of electric charge actually storable on the metal armatures of the capacitor.
To overcome this inherent limit of elastomeric polymers, composite materials are advantageously used in the electrostriction process, comprising an amorphous polymer base of elastomeric polymer film with the addition of a ceramic or metal conductive or conductive filler carbon black, nanoparticles of silver or copper). Said conductive filler must, however, be added to the polymer matrix in quantities such as to remain below the electrical percolation threshold (generally 15-20% by volume), in such a way that the composite material does not become electrically conductive, losing the dielectric capacity essential for the functioning of the system. Under these conditions, the particles of conductive filler become polarized due to the electric field applied to the capacitor armatures and contribute to increasing the amount of energy that the composite is able to convert.
Further drawbacks of such composite materials derive from the fact of not being cheap due to the presence of expensive metals, such as copper and silver, and to the fact of possessing a lower electrical resistance due to the presence of the conductive, filler which determines an increase in the dark current through the capacitor, thus introducing a constant energy loss over time.
Composite materials are also known of, in this type of applications, comprising an elastomeric polymer film base with the addition of an insulating filler material, typically ceramic, which has a higher relative dielectric constant c r compared to a conductive filler, with the function of improving the dielectric characteristics of the composite material.
A drawback typical of the latter category of composite materials is due to the fact that the interaction between the ceramic filler and the polymer matrix of the film is not very efficient and the amount of filler needed to achieve a significant increase in the electrical properties concerned is significant; consequently, the composite material is more rigid, brittle, and therefore susceptible to breakage or damage during the mechanical excitation needed for the production of electricity.
Yet a further drawback of these composite materials is that said filler materials are generally obtainable only using complex and expensive chemical processes that require special equipment or reactions involving hazardous reagents, harmful to health and polluting the environment.